Sending invasive rats packing

The tables are about to be turned on rodents that have been decimating bird populations in Alaska

By Erin Barnes

In the late 1700s, a Japanese sailing ship ran ashore one of Alaska’s 2,500 Aleutian Islands and there a few Norway rats jumped ship. It might have seemed like a harmless enough event, but it was the beginning of a long tale that Alaska’s Fish and Wildlife Service (FWS) hopes to bring to an end next winter.

In a word, the rats multiplied. And multiplied. Russian merchant vessels frequenting the islands in the 1800s brought more of them and then during World War II hundreds of military ships harboring the vermin visited the islands as well. The ships didn’t need to come ashore to spread the rodents; they’re able to swim for up to 72 hours. As the rats proliferated and spread to other islands in the chain, songbirds, seabirds, and other wildlife disappeared, becoming food for the vermin.

Today, rats abound on at least 21 islands in the Alaska Maritime National Wildlife Refuge, which encompasses the mostly uninhabited Aleutian Islands. Introduced rats are responsible for 40 to 60 percent of all reptile and bird extinctions in the refuge. "Rat spills" from the hundreds of ships entering Aleutian Island ports are more feared than oil spills because they devastate bird populations and threaten island biodiversity.

“We can’t control them. It’s just not realistic because their birth numbers are so high,” says Poppy Benson, the FWS public programs supervisor at the refuge. Wiping out all of the rats is the only way to stop them from overrunning the islands, she adds.

So, next winter FWS agents plan to fly 1,300 miles west of Anchorage to douse the birth place of the problem, now called Rat Island, with rodenticide in the hopes of eliminating the invasion; if the approach works there, the agency will expand it to other infested islands. In the next few weeks, the agency will finalize the eradication plan—a popular move among local citizens. Only 2 of the 37 comments submitted during the public comment period opposed the plan, and some respondents even offered to help kill the varmints.

The main reason for eradicated the rats is to protect the refuges’ birds, which have few defenses against the scavengers. Eighty-five percent of North America’s sea birds nest in the refuge, and of the 26 endemic bird species, some breed nowhere else in the world.

The danger the invasive species poses is perhaps most evident on Rat Island. “There are no song sparrows on Rat Island. That’s unheard of in an Aleutian Island, on an island of this size,” says Benson. “No sea birds at all.”

“Rats are burrow nesters. They dig underneath petrels and puffins [nests],” says Stacey Buckelew of Island Conservation. “So the rats have easy access, but the sea birds— with no anti-predation instincts—are very susceptible.”

On Kiska Island where some 4 million sea birds nest, rats catch and feed on baby birds in their nests. Ian Jones, a biologist at Memorial University in Newfoundland and Labrador, Canada, has predicted through population modeling that if this continues, the Least Auklets could be extinct in 50-100 years.

But birds aren’t the only creatures that feel the impact of rats’ voracious appetites. Because they’re opportunistic omnivores, when rats run out of birds to eat, they switch their diets, feeding instead on invertebrates like snails and small crabs.

Marine creatures can also be affected indirectly by rats, according to Carolyn Kurle, a grad student in Ecology and Evolutionary Biology at the University of California Santa Cruz. Glaucous-winged gulls and Black Oystercatchers eat herbivorous snails and other invertebrates. But Kurle found that when rats keep the numbers of these birds low, the invertebrate populations increase. These creatures, in turn, eat more algae, changing the makeup of the intertidal zone (the area exposed to air at low tide and submerged at high tide) from algae-dominated to invertebrate-dominated.

“It illustrates a pretty drastic indirect effect on a marine community by an invasive terrestrial predator,” says Kurle. “It helps conservation scientists to understand more completely all of the effects that invasive species can have on native species and habitats.”

Alaska has enacted strict laws to try to control the movement of rats. For example, it is illegal to house any of the animals on a boat within state-controlled waters. These measures are intended to prevent the spread of rats, but the populations that are already established just keep growing. Norway rats typically have four to six litters of six to twelve young each year; it’s estimated that one breeding pair can beget about 5,000 animals in one year.

That’s why the rats have to go, say wildlife managers.

Refuge managers have teamed up with Island Conservation, a science-based organization to protect island biodiversity, the Nature Conservancy, and experts from New Zealand’s Campbell Island rat eradication program—which, in the biggest effort to date, cleared the creatures from the 27,900-acre island. New Zealand is a nation of islands where critters have been slowly introduced, as happened in the Aleutian chain. But through eradication programs, over the past 25 years native wildlife has rebounded on 70 of 128 New Zealand islands plagued with invasive mammals.

Experts hope they can achieve similar success in the Aleutian Islands. Benson acknowledges that some birds that winter on the islands where rodenticide is dumped could be killed, “but that’s the price we’re willing to take in order to reestablish healthy [bird] populations,” she says. “There are so many bird species declining overall—it’s a wondrous opportunity to increase bird populations.”"